$(document).ready(function(){
$("#back").css({
    "height": function () {
    	if($(window).height()<450){
    		return 450;
    		}
    		else{
    		return ($(window).height());
    		}
                }
});
$("#gao-center").css({
    "marginTop": function () {
    return (($("#back").height()-$("#gao-center").height())/2+"px");
                }
});
});